1. The Problem: The Long‑Standing Challenge of Undercut Ejection
In parts with undercut features, conventional forced ejection often leads to a series of issues: surface scratches, sticking, stress whitening, and even direct scrap.
The root cause is simple: the part is “pulled” out before it has fully released from the mold.
Is there a way to actively create clearance for the part before ejection?
The answer is a collapsing core.
2. The Solution: A 6‑Segment Collapsing Core Structure
In this mold, we adopted a 6‑segment collapsing core (3 large + 3 small segments).
How it works is straightforward:
- During molding: the segments expand outward to form the full undercut shape
- Before ejection: the segments collapse inward, creating clearance for the part
- During ejection: the part is released naturally, without forced extraction
The video below shows the smooth “expand & collapse” motion of this collapsing core. Even in a relatively tight mold space, this structure operates reliably:
3. Material: Why S136?
A collapsing core must not only move smoothly but also withstand long‑term production.
For this mold core, we chose S136 imported stainless steel for three key reasons:
- Excellent mirror polish – the surface in contact with the part is smooth, reducing scratch risk
- Strong corrosion resistance – resists rust during long production runs, suitable for high‑cleanliness applications
- Long service life – high hardness and wear resistance, ideal for high‑volume production
4. Results: Proven in Mass Production
This mold has been validated through 300,000 shots cycles production:
- Collapsing core operates smoothly with no sticking
- No scratches, no sticking on the part surface
5. Typical Applications
This collapsing core structure is particularly suitable for:
- Parts with deep or complex undercuts
- High surface quality requirements (e.g., cosmetic parts, transparent parts)
- High‑volume production demanding long mold life and consistent stability
6. Struggling with Undercut Ejection?
If you are facing undercut ejection problems, let’s talk. We can explore together whether a collapsing core solution can help improve your yield rate.
📩 Contact us: sales1@rilong-mold.com
